There are two different drugs in this study. One is called Calcitriol and is approved by the Food and Drug Administration (FDA) as a vitamin D supplement. The other drug is called cholecalciferol and it is approved by the FDA. Unlike calcitriol, cholecalciferol is a nutritional form of vitamin D that can often be found in various types of foods and its chemical structure must be changed by the body to become the active form of vitamin D. It is believed that cholecalciferol may have different effects in the body compared to calcitriol. It is possible that CKD patients would benefit from receiving both of these drugs, but this is currently unclear. While on this study you will receive either one of these study drugs, depending on which group you are assigned to.